1. Field of the Invention
The present invention relates generally to electromagnetic energy radiation and reception, and especially relates to electromagnetic energy radiation and reception effected using impulse radio energy. Still more particularly the present invention provides an antenna suited for broadband energy radiation and reception, and particularly well suited for broadband energy radiation and reception employing impulse radio energy.
2. Related Art
Recent advances in communications technology have enabled an emerging revolutionary ultra wideband technology (UWB) called impulse radio communications systems (hereinafter called impulse radio).
Impulse radio was first fully described in a series of patents, including U.S. Pat. No. 4,641,317 (issued Feb. 3, 1987), U.S. Pat. No. 4,813,057 (issued Mar. 14, 1989), U.S. Pat. No. 4,979,186 (issued Dec. 18, 1990) and U.S. Pat. No. 5,363,108 (issued Nov. 8, 1994) to Larry W. Fullerton. A second generation of impulse radio patents include U.S. Pat. No. 5,677,927 (issued Oct. 14, 1997) to Fullerton et al; and U.S. Pat. No. 5,687,169 (issued Nov. 11, 1997) and U.S. Pat. No. 5,832,035 (issued Nov. 3, 1998) to Fullerton. These patent documents are incorporated herein by reference.
Uses of impulse radio systems are described in U.S. patent application Ser. No. 09/332,502, entitled, “System and Method for Intrusion Detection Using a Time Domain Radar Array,” and U.S. patent application Ser. No. 09/332,503, entitled, “Wide Area Time Domain Radar Array,” both filed Jun. 14, 1999, both of which are assigned to the assignee of the present invention, and both of which are incorporated herein by reference.
Basic impulse radio transmitters emit short pulses approaching a Gaussian monocycle with tightly controlled pulse-to-pulse intervals. Impulse radio systems typically use pulse position modulation, which is a form of time modulation where the value of each instantaneous sample of a modulating signal is caused to modulate the position of a pulse in time.
For impulse radio communications, the pulse-to-pulse interval is varied on a pulse-by-pulse basis by two components: an information component and a pseudo-random code component. Unlike direct sequence spread spectrum systems, the pseudo-random code for impulse radio communications is not necessary for energy spreading because the monocycle pulses themselves have an inherently wide bandwidth. Instead, the pseudo-random code of an impulse radio system is used for channelization, energy smoothing in the frequency domain and for interference suppression.
Generally speaking, an impulse radio receiver is a direct conversion receiver with a cross correlator front end. The front end coherently converts an electromagnetic pulse train of monocycle pulses to a baseband signal in a single stage. The data rate of the impulse radio transmission is typically a fraction of the periodic timing signal used as a time base. Because each data bit modulates the time position of many pulses of the periodic timing signal, this yields a modulated, coded timing signal that comprises a train of identically shaped pulses for each single data bit. The impulse radio receiver integrates multiple pulses to recover the transmitted information.
In a multi-user environment, impulse radio depends, in part, on processing gain to achieve rejection of unwanted signals. Because of the extremely high processing gain achievable with impulse radio, much higher dynamic ranges are possible than are commonly achieved with other spread spectrum methods, some of which must use power control in order to have a viable system. Further, if power is kept to a minimum in an impulse radio system, this will allow closer operation in co-site or nearly co-site situations where two impulse radios must operate concurrently, or where an impulse radio and a narrow band radio must operate close by one another and share the same band.
Many applications for impulse radio technology, including communication applications, position determination applications, locating (e.g., radar) applications and other applications require lightweight, compact broadband antennas with omni-directional transmit/receive characteristics. As with any antenna, impedance matching to feed elements is necessary for efficient operation. Moreover, in the case of impulse radio technology applications, the antenna must not be subject to ringing in response to application of pulses—either in a transmit mode or in a receive mode.
Current antenna technology offers several undesirable alternatives to one interested in a small, well-matched, efficient, omni-directional ultra wideband (UWB) short pulse antenna: (1) a self-similar antenna (e.g., a bow tie antenna) that tends to be large and frequency dispersive; (2) a TEM horn antenna that tends to be bulky and highly directive; or (3) a resistively loaded antenna that will necessarily be lossy and inefficient. Existing spheroidal antennas like the volcano smoke antenna (FIG. 10) are difficult to manufacture. Existing UWB antennas like the biconical antenna are relatively large and, despite their stable impedance, are not well matched to 50Ω.
Kraus (John D. Kraus, Antennas, 2nd edition; New York: McGraw Hill, 1988) briefly mentions a “double dish” antenna comprised of a pair of hemispherical dishes connected in tandem to form a dipole with planar elements facing away from each other. (Kraus; p. 63) The “double dish” configuration is presented as a step in evolving an antenna configuration from Kraus's “volcano smoke” antenna (FIG. 10) to a stub (monopole) antenna. Kraus' “double dish” antenna does not meet the performance criteria recognized herein as necessary for optimal performance in an impulse radio application. The sharp discontinuities in transitioning from a smooth curve to a substantially planar outwardly facing dish surface creates undesirable internal reflections in the “double dish” antenna.
The current art regarding ultra wideband (UWB) antennas teaches using element antennas such as monopoles, dipoles, conical antennas and bow-tie antennas for ultra wideband systems. However, they are generally characterized by low directivity and relatively limited bandwidth unless either end loading or distributed loading techniques are employed, in which case bandwidth is increased at the expense of radiation efficiency.
Conventional antennas are designed to radiate only over the relatively narrow range of frequencies used in conventional narrow band systems. Such narrow band systems may, for example, employ fractional bandwidths no more than about 25%. If an impulse signal, such as a signal of the sort employed for impulse radio purposes, is fed to such a narrow band antenna, the antenna tends to ring. Ringing severely distorts signal pulses and spreads them out in time. Impulse radio signals are preferably modulated by pulse timing, so such distortion of pulses is not desirable.
Broadband antennas are advantageous for many purposes, including their use with impulse radio systems. Conventional design in broadband antennas follows a commonly accepted principle that the impedance and pattern properties of an antenna will be frequency independent if the antenna shape is specified only in terms of angles. That is to say, a self-similar or self-complimentary antenna will be a broadband antenna. This principle explains known broadband antennas like biconical and bow tie antennas, but also applies to other broadband antennas like log periodic, log spiral, and conical spiral antennas.
All such prior art antennas rely on a variation of scale to achieve their broadband performance. A “smaller” scale section of the antenna radiates higher frequency components while a “larger” scale section of the antenna radiates lower frequency components. Because the radiation centers change location as a function of frequency, these antennas are inherently frequency dispersive; they radiate different frequency components from different parts of the antenna, resulting in a distorted impulse signal.
Throughout this description, it should be kept in mind that discussions relating to transmitting or transmissions apply with equal veracity to reception of electromagnetic energy or signals. In order to avoid prolixity, the present description will focus primarily on transmission characteristics of antennas, with the proviso that it is understood that reception of energy or signals is also inherently described.
A biconical antenna is a classic example of a prior art broadband antenna with an omni-directional pattern. A typical biconical antenna with a 60° half angle will have a 100Ω input with a voltage standing wave ratio (VSWR) of <2:1 over a 6:1 bandwidth. A significant drawback with such a biconical antenna is that such an antenna is typically implemented with a diameter equal to the wavelength at the lower frequency limit (λI,) thus requiring that the antenna be 0.577λL in height. Because of similar design limitations, a typical monocone antenna will not provide a good match if it is much less than 0.22λL in diameter. In any event, a monocone antenna does not have very stable performance over a broad band. Antennas as large as the above described typical conical antennas (biconical and monoconical) often have difficulty radiating (i.e., transmitting) pulses without dispersion. In addition, such large antennas are difficult to fit into a small portable or hand held devices.
TEM horn antennas often suffer from frequency dispersion as well. Furthermore, a horn antenna is inherently a large structure, often several wavelengths in dimension. A horn antenna may be made smaller by dielectric loading, but such loading adds weight which is often undesirable. Further, a horn antenna is a directive antenna and cannot provide the omni-directional coverage required for many portable or mobile applications.
A TEM feed may be combined with a parabolic dish to create a ribbed horn “impulse radiating antenna” (IRA). Such antennas can have bandwidths on the order of a couple of decades, and very high gain, but their large size and high directivity make them inappropriate for portable or mobile use.
A “dish” antenna consisting of the rounded sides of two spherical hemispheres being driven against one another is a known antenna structure (e.g., Kraus' “double dish” antenna), but it is not known to be used for impulse radio broadband applications. Another prior art attempt to provide a spheroidal antenna is a “volcano smoke” antenna (see, Kraus; p. 63). The tapered feed of this antenna provides excellent matching, and the antenna does radiate omni-directionally, but the gradual transition required to yield such beneficial operating parameters makes the antenna bulky and difficult to manufacture.
Because spherical antennas must be fed by a radial waveguide, they often exhibit poor matching characteristics unless an elaborate and difficult-to-manufacture impedance matching structure is used. An impedance matching structure also tends to further impair antenna performance by making the antenna more likely to ring. It is very difficult to construct a feed that maintains a constant matched impedance over a broad bandwidth, something essential to an ultra wideband (UWB) antenna. It is a commonly accepted design criteria in electromagnetic applications, and especially in radio communication applications, that an antenna should match a 50Ω impedance feed providing signals to (or receiving signals from) the antenna. Some video applications require matching a 75Ω impedance feed.
Another prior known antenna structure drives a hemispherical antenna against a ground plane. Attempts by the inventor to employ such an antenna structure for broadband impulse radio resulted in an unacceptably large impedance mismatch.
Circular disc (planar) monopole antennas and elliptical disc (planar) monopole antennas have been evaluated to determine their respective bandwidths. (Agrawall, Kumar and Ray; “Wide-Band Planar Monopole Antennas”; IEEE Transactions on Antennas and Propagation, February 1998.) However, no regard was given to the suitability of such antennas for impulse radio applications. No regard was given to dispersion, ringing or phase performance of signals employing such circular disc antennas or elliptical disc antennas for impulse radio communication.
Resistive loading is an alternate technique commonly employed to achieve impedance matching in broadband antennas. Resistive loading succeeds in reducing reflection, but at the cost of throwing away typically around half the power that may be transmitted by an antenna. Such a design trade-off has become accepted in design approaches in prior art antennas. It has been generally believed that resistive loading must be employed for a small broadband antenna in order to achieve good impedance matching. Non-resistively loaded small ultra wideband antennas are known, but they tend to have poor impedance matching and high voltage standing wave ratios (VSWR's). A lower value for VSWR is a better value; the optimum value of VSWR is 1:1. The prior art teaches that resistive loading must be used in an element antenna in order to achieve wide bandwidth. It is commonly believed that high radiation efficiency and high bandwidth are mutually exclusive.
For a small hand held or portable system, it is desirable to have a well matched, efficient, physically small, UWB antenna that radiates non-dispersively and omni-directionally. It is particularly advantageous for an antenna to be easily made in large volumes with reliable repeatable quality. Not only are such antennas unknown to the present art, in fact, the current teaching is that such antennas are not physically realizable.
There is a need for a broadband antenna that is compact, efficiently matched to a feed structure and radiates omni-directionally.
In particular, there is a need for a broadband antenna that operates without ringing in response to application of a pulse signal.